This invention relates to a capacitor having a large capacity per unit weight and good leakage current (hereinafter abbreviated to as xe2x80x9cLCxe2x80x9d) characteristics.
By virtue of advancement in the downsizing or higher integration of IC or printed board in recent years, compact and lightweight electronic instruments such as a portable telephone, a laptop personal computer and an electronic memorandum book, have come into wide use. To cope with this tendency, development of capacitors having a small size and a large capacity is being eagerly desired for use in these electronic instruments.
Among the capacitors used in this field, a tantalum electrolytic capacitor is widely used because it has a large capacity for the size and exhibits good performance. In this tantalum electrolytic capacitor, tantalum oxide is used for the dielectric material.
In order to more increase the capacity of a capacitor, development of niobium or titanium capacitors using niobium oxide or titanium oxide having a higher dielectric constant than the tantalum oxide for the dielectric material is being encouraged. However, the capacitors using niobium oxide or titanium oxide for the dielectric material have unsatisfactory LC characteristics and poor practicality, thus, they are still in need of improvements.
An object of the present invention is to provide a niobium capacitor provided with a niobium oxide dielectric having good dielectric properties, which capacitor has a large capacity per unit weight and good LC characteristics.
Another object of the present invention is to provide a process for producing a niobium capacitor having a large capacity per unit weight and good LC characteristics, and exhibiting uniform LC value.
The present inventors have found that the poor LC characteristics of the niobium capacitor provided with a niobium oxide dielectric are, as one of causes, due to the excessive or deficient amount of oxygen bound to niobium constituting the niobium oxide. The present invention has been accomplished based on this finding.
In accordance with the present invention, there is provided a capacitor comprising two electrodes and a dielectric interposed between the two electrodes, characterized in that the dielectric has a two-layer structure comprising a first layer predominantly comprised of niobium oxide NbOx (X=2.5) and a second layer predominantly comprised of a mixture of niobium oxide NbOx (X=2.5) and niobium oxide NbOx (X=2.0).
In accordance with the present invention, there is further provided a process for producing a capacitor comprising two electrodes, one of which Is comprised of a sintered body of partially nitrided niobium, and a dielectric interposed between the two electrodes, characterized in that a compact of powdery niobium is sintered and then the thus-obtained niobium sintered body is allowed to stand in a nitrogen atmosphere to partially nitride the niobium sintered body.
In the niobium capacitor of the present invention, the dielectric interposed between two electrodes is a dielectric having a two-layer structure comprising a first layer predominantly comprised of niobium oxide NbOx (X=2.5) and a second layer predominantly comprised of a mixture of niobium oxide NbOx (X=2.5) and niobium oxide NbOx (X=2.0).
In general, if the structure of niobium oxide is expressed by the formula: NbOx (x represents a molar ratio of oxygen bonded to niobium), those where x is 0.9, 1.0, 1.1, 2.0 and 2.5 are known. Niobium oxides having such a bonding value in this structure are identified by the X-ray photoelectric spectroscopic analysis. As preferable examples of niobium oxide NbOx (x=2.5) and niobium oxide NbOx (x=2.0), there can be mentioned Nb2O5 and NbO2, respectively.
When the dielectric in a capacitor is constituted by a two layer structure having a first layer predominantly comprised of niobium oxide NbOx (X=2.5) and a second layer predominantly comprised of a mixture of niobium oxide NbOx (X=2.5) and niobium oxide NbOx (X=2.0), among niobium oxides, the capacitor has a very low LC value. The reason therefor is not yet completely elucidated, however, the poor LC characteristics are presumed to result because when a dielectric is predominantly comprised of niobium oxide, which does not have the above-described two-layer structure, oxygen in the dielectric material moves from the dielectric side to the electrode side or internal oxygen adsorbed on the electrode moves from the electrode side to the dielectric side, and due to this moving of oxygen, the characteristics of the dielectric itself become unstable, leading to an increase of the LC value. On the other hand, when a niobium oxide dielectric having the above-descried two-layer structure is used as dielectric, it is considered that the movement of oxygen, even if it occurs, takes place inside the dielectric material and the state is seemingly equilibrated, as a result, characteristics of the dielectric itself can be stabilized.
By the term xe2x80x9cpredominantly comprised ofxe2x80x9d used in the niobium oxide dielectric used in the present invention, we mean that niobium oxide NbOx (X=2.5) occupies at least 60% by weight of the first layer, and the mixture of niobium oxide NbOx (X=2.5) and niobium oxide NbOx (X=2.0) occupies at least 60% by weight of the second layer. The LC value of a capacitor is preferably 1 xcexcA or less. In order to keep the LC value at 1 xcexcA or less, the content of NbOx (x=2.5) in the niobium oxide of the first layer and the content of the mixture of NbOx (x=2.5) and NbOx (x=2.0) in the second layer each should preferably be at least 90% by weight, more preferably at least 95% by weight.
To produce a capacitor having a more reduced LC value, the ratio of niobium oxide NbOx (X=2.5) to niobium oxide NbOx (X=2.0), contained in the second layer of the dielectric, is preferably in the range of 1:4 to 4:1 by mole, more preferably from 1:3 to 3:1 by mole; and the content of the first layer in the two-layer structure Is preferably in the range of 0.01% to 10% by volume, especially 0.04% to 3% by volume, based on the volume of the second layer.
For forming the niobium oxide dielectric layer having the above-mentioned two-layer structure, for example, there can be employed a method of depositing a niobium complex such as niobium-containing alkoxy complex or acetyl acetonate complex onto an electrode, and thermally decomposing and/or hydrolyzing the deposited niobium complex; or, in the case of using niobium or partially nitrided niobium for the electrode, which will be described later, a method of electrolytically oxidizing the niobium electrode or the partially nitrided niobium electrode, or a method of depositing a niobium complex, as mentioned above, onto the niobium electrode or the partially nitrided niobium electrode and thermally decomposing and/or hydrolyzing the deposited niobium complex. Depending on the case, these methods may be used in combination.
In the case where the niobium oxide dielectric is made by electrolytically oxidizing the niobium electrode or the partially nitrided niobium electrode, the capacitor of the present invention is an electrolytic capacitor wherein the niobium electrode or the partially nitrided niobium electrode assumes anode. In the case where the niobium oxide dielectric is made by decomposing a niobium complex on the niobium electrode or the partially nitrided niobium electrode, the electrode is theoretically free of polarity and may assume either anode or cathode.
For the electrolytic oxidation of the niobium electrode or the partially nitrided niobium electrode, an aqueous protonic acid solution, for example, a 0.1% aqueous phosphoric acid solution or a 0.1% aqueous sulfuric acid solution is usually used. When the niobium oxide dielectric is made by the method of thermally decomposing and/or hydrolyzing a niobium-containing complex, the conditions such as the kind and concentration of the niobium complex, the decomposition temperature, the decomposition time and the kind and concentration of gas in the decomposition atmosphere, or by the method of electrolytically oxidizing the niobium electrode or the partially nitrided niobium electrode, the conditions such as the kind and shape of the electrode used, the kind and concentration of the electrolytic solution, and the electrolysis temperature and time, must be determined by previously examining an X-ray photoelectron spectroscopic diagram of the dielectric, obtained in a preliminary test. This is because the value X in the formula NbOx varies depending upon the above-recited conditions.
In general, there is a tendency that as the decomposition temperature is higher, as the decomposition time is longer, as the oxygen gas concentration in the gas of decomposition atmosphere is higher, as the concentration of the electrolytic solution is higher, as the electrolytic temperature is higher or as the decomposition time Is longer, the value x of niobium oxide NbOx obtained is lager.
The dielectric used in the present invention exhibits the desired function provided that it is interposed between the two electrodes. The shape and other structural features are not particularly limited. The thickness of the dielectric must not be uniform. The dielectric may have a part of complicated shape such that the electrodes are combined therewith in an intricate configuration.
As examples of the material for one electrode used in the capacitor of the present invention, there can be mentioned aluminum, tantalum, titanium, niobium, niobium nitride obtained by nitriding a part of niobium, and alloys of these metals.
Examples of the electrode shape include sheet, foil, bar and sintered body. The size of the capacitor is determined depending upon the required capacity of the capacitor. In the case of sheet, foil or bar, the electrode is used after bending or coiling it to increase the surface area per unit area. In the case of a sintered body, the electrode may be formed by compacting fine powder of the above-described metal under pressure and sintering the thus-prepared compact at a temperature of from 500xc2x0 C. to 2,000xc2x0 C. and a reduced pressure of from 100 Torr to 10xe2x88x926 Torr for from several minutes to several hours.
Niobium or partially nitrided niobium is preferably used as the electrode material, because a capacitor having a large capacity per unit weight is obtained. Especially the partially nitrided niobium is more preferably used, because good LC characteristics are additionally obtained. Accordingly, a niobium capacitor having an electrode comprised of partially nitrided niobium Is suitably used as a circuit capacitor required to have a high voltage and a low LC.
The partially nitrided niobium is obtained by partially nitriding niobium, for example, in a nitrogen atmosphere. The content of bound-nitrogen in the partially nitrided niobium varies depending on the shape of the niobium metal, however, in the case of powder having a particle diameter of approximately 30 xcexcm or smaller, it is in the range of from 10 ppm to 200,000 ppm, preferably from 10 ppm to 150,000 ppm and more preferably 100 ppm to 10,000 ppm by weight, based on the weight of the partially nitrided niobium.
The reaction temperature for nitriding is not particularly limited, however, partially nitrided niobium having a necessary bound-nitrogen content may be industrially obtained by nitriding at a temperature of from room temperature to 2,000xc2x0 C., preferably from 250 to 2,000xc2x0 C. for approximately from 1 to 50 hours. In general, as the temperature is higher, the surface can be nitrided within a shorter time. Even at a low temperature of about room temperature, when fine powder of niobium metal Is left standing for tens of hours or longer in a nitrogen atmosphere, partially nitrided niobium having a necessary bound-nitrogen content of from tens of ppm to hundreds of ppm can be obtained.
In the case where an electrode comprised of a partially nitrided niobium sintered body is made, there can be employed a method of partially nitriding a niobium powder or its compact and then sintering the partially nitrided niobium, and a method of sintering a compact of niobium powder and then partially nitriding the niobium sintered body. The latter method of conducting first sintering and then nitriding is preferable because capacitors having uniform LC values can be obtained. That is, when a niobium powder is first partially nitrided and then the partially nitrided niobium is sintered according to the former method, the microstructure of the resulting electrode is sometimes not uniform due to heating of partially nitrided niobium upon sintering. In contrast, when a compact of niobium powder is first sintered and then the sintered body is partially nirided, the resulting electrode has a uniform microstructure and the non-uniformity of LC values of capacitors is reduced.
A sintered body of non-nitrided niobium may be obtained, for example, by sintering a compact of powdery niobium at a high temperature in vacuum. More specifically, powdery niobium is molded into a compact and then the compact is allowed to stand under a reduced pressure of from 10xe2x88x921 to 10xe2x88x926 Torr at a temperature of from 1,000 to 2,000xc2x0 C. for from a few minutes to several hours. The sintering temperature generally varies depending on the particle diameter of powdery niobium and as the particle diameter is smaller, a lower temperature may be used.
The conditions under which a niobium sintered body is partially nitrided, and the content of bound-nitrogen in the partially nitrided niobium may be the same as those mentioned as for partially nitriding of powdery niobium. In general, a niobium sintered body having an objective content of bound-nitrogen can be obtained by partially nitriding at a temperature of 2,000xc2x0 C. or lower for a time of tens of hours. In general, nitriding at a higher temperature may be completed within a shorter time. Even at room temperature, when the niobium sintered body is left standing for tens of hours in a nitrogen atmosphere, a niobium sintered body having a bound-nitrogen content of hundreds of ppm by weight can be obtained. By introducing nitrogen under pressure, the nitriding time can be shortened. On the contrary, when nitrogen is introduced under reduced pressure, the nitriding time is prolonged. For example, if the niobium sintered body is left standing under extremely reduced pressure, e.g., {fraction (1/100)} Torr, nitriding scarcely takes place within an industrially acceptable time of tens of hours.
As mentioned above, capacitors having an electrode comprised of partially nitrided niobium sintered body, which has been prepared by a method of conducting first sintering and then nitriding, exhibit reduced non-uniformity of LC values. This effect of reduction in non-uniformity of LC values is found not only when the dielectric of the capacitors is comprised of niobium oxide, but also when the dielectric is comprised of other materials such as tantalum oxides, polymeric materials and ceramic materials. As examples of such materials used for the dielectric, other than niobium oxide, there can be mentioned tantalum oxide derivatives such as those which are prepared by depositing a tantalum-containing complex, for example, an alkoxy complex of tantalum or an acetylacetonate complex of tantalum, to the electrode and then hydrolyzing and/or thermally decomposing the deposited complex; polymeric materials which include, for example, fluororesins, alkyd resins, acrylic resins, polyester resins such as polyethylene terephthalate, vinyl resins, xylylene resins and phenolic resins; and ceramic dielectric materials which include, for example, perovskite-type compounds such as BaTiO3, SrTiO3 and BaSnO3, formed on the surface of a metal having pores or voids as described, for example, in JP-A 7-63045.
The other electrode in the capacitor of the present invention is not particularly limited. For example, at least one compound selected from electrolytic solutions, organic conducting materials derived from organic semiconductors and inorganic conducting materials derived from inorganic semiconductors, which are known in the art of aluminum electrolytic capacitors, may be used. At least one organic semiconductor or inorganic semiconductor is preferably used for the other electrode, which preferably has an electrical conductivity of from 10xe2x88x922 Sxc2x7cmxe2x88x921 to 103 Sxc2x7cmxe2x88x921. When an organic or inorganic semiconductor having an electrical conductivity of from 10xe2x88x922 Sxc2x7cmxe2x88x921 to 103 Sxc2x7cmxe2x88x921 is used, the impedance value of a capacitor can be more reduced and the capacity thereof at a high frequency can be more enhanced.
Examples of the organic semiconductor include an organic L semiconductor comprising benzopyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane, and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant to a polymer represented by the following general formula (1) or (2): 
(wherein R1 to R4 each represents hydrogen, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R5 is present only when X is a nitrogen atom and represents hydrogen or an alkyl group having 1 to 6 carbon atoms, and R1 and R2 or R3 and R4 may be combined with each other to form a ring together with the carbon atoms on the benzene ring, to which R1 and R2 or R3 and R4 are bound. 
(wherein R1 and R2 each represents hydrogen, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R3 is present only when X is a nitrogen atom and represents hydrogen or an alkyl group having 1 to 6 carbon atoms, and R1 and R2 may be combined with each other to form a ring together with the carbon atoms on the five-membered ring, to which R1 and R2 are bound.).
As specific examples of the electrically conducting polymer represented by formula (1) or (2), there can be mentioned polyaniline, polyoxyphenylene, polyphenylenesulfide, polythiophene, polyfuran, polypyrrole and polymethylpyrrole.
Examples of the inorganic semiconductor include an inorganic semiconductor mainly comprising lead dioxide or manganese dioxide, and an inorganic semiconductor comprising triiron tetroxide.
These semiconductors may be used either alone or in combination of two or more thereof.
In the case when the other electrode is a solid, a capacitor can be fabricated, for example, by sequentially laminating a carbon paste and a silver paste on the other electrode and encapsulating the laminate with a material such as epoxy resin This capacitor may have a niobium or tantalum lead which is formed by sintering together with the niobium sintered body or by afterward welding. In the case where the other electrode is a liquid, a capacitor can be fabricated, for example, by housing a structure comprising the above-mentioned electrode and a dielectric in a can electrically connected to the other electrode. In this case, the partially nitrided niobium sintered body electrode side is guided outside through the niobium or tantalum lead and at the same time, designed to be insulated from the can and the other electrode by using an insulating rubber or other insulating materials. In the capacitor, there may be present a portion where the dielectric is incompletely connected to the electrode, i.e., the dielectric material is partly not in contact with the electrode.
The present invention will now be more specifically described by the following examples.
Characteristics of powdery niobium, a niobium sintered body and a capacitor were determined by the following methods.
(1) Average Particle Diameter of Powder
Average particle diameter (unit: xcexcm) of a niobium powder was expressed by a particle diameter value D50 as determined at a cumulative weight of 50% by a particle size distribution analyzer (tradename xe2x80x9cMicrotrackxe2x80x9d).
(2) Content of Bound-Nitrogen
The content of bound-nitrogen in a niobium powder or a niobium sintered body was determined by using an oxygen-nitrogen analyzer (available from LECO Co.) measuring a nitrogen content based on the thermal conductivity.
(3) Capacity of Capacitor
Capacity (unit: xcexcF) of a capacitor was determined at a frequency of 120 Hz in Examples 1 to 15 or 100 kHz in Examples 16 to 36 by an LCR measuring device (available from HP Co.), a terminal of which was directly connected to an electrode of the capacitor.
(4) Leakage Current (LC) Value of Capacitor
Leakage current (LC) value (unit: xcexcA) of a capacitor was measured by a leakage current measuring device, a terminal of which was directly connected to an electrode of the capacitor, when one minute elapsed while a voltage of 4 V was imposed. The LC value was measured on 20 capacitors and expressed by an average value.
(5) Non-uniformity (2 "sgr") of Leakage Current Value
Average value and standard deviation ("sgr") of LC values were determined for 20 specimens, and non-uniformity of LC value was expressed by a doubled standard deviation value (2 "sgr").